An EL device display can be classified according to its driving method, passive matrix (PM-EL Display) and active matrix (AM-EL Display). AM-EL display uses TFT (Thin Film Transistor) with a capacitor for storing data signals that can control EL display gray levels of brightness.
The manufacturing procedure of a PM-EL display is simpler in comparison and less costly; however, it is limited in its size (<5 inches) because of its driving mode and has a lower-resolution display application. In order to produce an EL display with higher resolution and larger size, utilizing active-matrix driving is necessary. The AM-EL uses TFT with a capacitor for storing data signals, so that the pixels can maintain their brightness after line scanning; on the other hand, pixels of passive matrix drive only light up when the scan line selects them. Therefore, with active matrix driving, the brightness of EL device is not necessarily ultra-bright, resulting in longer lifetime, higher efficiency and higher resolution. Naturally, EL devices with active matrix driving are suitable for display applications of a higher resolution and excellent picture quality.
LTPS (Low Temperature Poly-Silicon) and a-Si (Amorphous Silicon) are both technologies of TFT integrating on glass substrate. The obvious differences are electric characteristics and complexity of processing. Although LTPS-TFT possesses higher carrier mobility and higher mobility means more current can be supplied, the process is much more complex. However, the process of a-Si TFT is simpler and more mature, except for low carrier mobility. Therefore, a-Si process has better competitive advantage in cost.
Due to limitations of LTPS process capability, threshold voltage (Vth) and mobility of TFT elements produced vary leading to different properties of each TFT element. When the driving system achieves gray scale by analog voltage modulation, an EL device produces a different output current despite having the same data voltage signal input due to the different TFT characteristics of various pixels. Therefore, the luminance of an OLED varies. Images of erroneous gray scale will show up on OLED panel and seriously damage image uniformity.
The most urgent problem of the AM-EL display to be solved currently is how to reduce the impact of uneven LTPS-TFT characteristics. Such an issue requires an immediate solution for follow-up development and applications since images on the display tell the difference.
U.S. Pat. No. 6,373,454 discloses ┌Active matrix electroluminescent display devices┘ (Apr. 26, 2002), No. 6,229,506 discloses ┌Active matrix light emitting diode pixel structure and concomitant method┘ (May 26, 2001) and Toshiba publishes a thesis titled ┌A Novel Current Programmed Pixel for Active Matrix OLED Displays┘ (Society for Information Display 2003 (SID 2003)).
For the above patents and thesis, the input current on the data line and output current to the EL device was 1:1. Thus, there was a defect of long charge/discharge times for the capacitor and the parasitical capacitor at low current input.
U.S. Pat. No. 6,359,605 discloses ┌Active matrix electroluminescent display devices┘ (Mar. 26, 2002), U.S. Pat. No. 6,501,466 proposes ┌Active matrix type display apparatus and drive circuit thereof┘ (Dec. 26, 2002) and U.S. Pat. No. 6,535,185 presents ┌Active driving circuit for display panel┘ (Mar. 26, 2003).
For the aforementioned patents, the theory of current mirror is utilized to achieve the rate of input current and output current as n:1. However, two TFTs in the current mirror have to be matched to prevent threshold voltage (Vth) difference and mobility. Thus, requirements for TFT manufacturing process are stricter.
The common problem for the circuits above is the voltage discharged from the storage capacitor to the gate and source of the driving transistor (Vgs) has to be less than the threshold voltage (Vth) of the driving transistor. Nevertheless, Vgs is larger than Vth due to long discharging time. Consequently, EL device illuminates via a small current and therefore the contrast of the display panel is not good.
A thesis with the subject of ┌A New Current Programmable Pixel Structure for large-Size and High-Resolution AMOLEDs┘ is released by Samsung (International Display Workshops 2002 (IDW 2002)). The theory of capacitive coupling is applied to change the gate voltage (Vg) of the driving TFT to establish a relationship between the output and input currents as output current=A×input current+B (A and B are constants). As capacitors are affected by the process or the layout, the voltage of capacitive coupling changes and the output current of the driving transistor is influenced, too. Defects of this driving method are capacitance precision and impose strict requirements for capacitor processing and layout. In addition, the aperture ratio of pixels becomes smaller as two capacitors are required to drive one pixel.